It is well known that faults in optical fibres can be located by an OTDR (optical time domain reflectometer). An OTDR launches a pulse of light into a fibre, and backscattered light is monitored for abrupt changes indicative of a fault, the time between pulse launch and the detection of the light at the launch end being indicative of the distance along the fibre that the fault occurs. In addition to back scattered light, there may be a Fresnel reflection from the far end of the fibre. Suggestions have been made to use reflection of an outbound pulse, (see Electronics Letters Vol. 20 No. 8, Gold, Hartog and Payne), regarding it as equivalent to launching a pulse at the far end of the fibre, and monitoring the backscattered light resulting from the reflection to give, in combination with the monitoring of the backscattering from the outbound pulse, in effect a simulated, double-ended OTDR. However, no suggestion has been made to monitor the reflection peak itself to provide information relating to losses.
In branched networks, it would be extremely costly to monitor each line separately from the most diversified end. It is, therefore, desirable to be able to monitor the branch lines from a centralised location, such as an exchange. If an OTDR is used, then the backscattered light from each branch line is combined on its return to the junction of the branches, and it is not possible to determine from which branch line it originated, although a distance from the pulse source is known. Also, in a branched network, the power of the outbound pulse is divided into the branch lines. Thus, the information relating to any branch line has only a strength related to the backscattering from the portion of the pulse in that branch line superimposed on the information from all the other branch lines, which will decrease the resolution thereby reducing the dynamic range of the instrument and sensitivity of attenuation measurement in a particular branch line. In general, an OTDR at present has a backscatter range limitation of approximately 20 dB for a 100 ns pulse width. Thus, if the branch lines are of substantial length and/or diversity, it may not be possible to monitor the entire network by normal OTDR methods. In any event, specific branch line(s) at fault would not be identified.
The aim of the present invention is to enable individual monitoring of branch lines in a branched network upstream from the branching point.